Dry lubricant drilling of thru-holes in printed circuit boards

Abstract

Thru-holes are drilled in a printed circuit board panel having alternating dielectric layers and electrically conductive layers. A water soluble dry film lubricant sheet is placed adjacent an electrically conductive outer layer of the panel prior to drilling thru-holes through the panel with a rotary drill. The drill surface is lubricated while forming a clean thru-hole from contact between the drill and the lubricant. The dry film lubricant generally may comprise a water soluble lubricant hardened to a solid or semi-solid dry film-form and impregnated in a carrier such as a porous paper sheet. In one embodiment, the dry film lubricant includes a water soluble lubricant such as dipropylene glycol, a non-ionic surfactant, and a wax-like hardener. Following drilling of the thru-holes, the dry film lubricant sheet is removed, and the thru-holes can be rinsed in an aqueous alkaline solution. The thru-holes are then plated with an electrically conductive metal for forming electrical contacts between the metalized thru-holes and the electrically conductive layers of the panel. The dry film lubricant extends drill life and produces a clean thru-hole that prevents epoxy smear and avoids the prior art use of harsh chemical treatment following drilling to otherwise remove dielectric smear or fibers that might extend into the thru-hole and disrupt the continuity of the later-formed electrical contact.

What is claimed is: 
     
       1. In a process for drilling printed circuit boards in a dry state which includes drilling a thru-hole in a printed circuit board panel having a dielectric layer made from a material which can be smeared onto other portions of the circuit board panel by the drilling tool and thereby interfere with subsequent electrical performance of the finished circuit board, an improvement for reducing dielectric smear comprising the steps of: placing a water soluble and substantially dry lubricating film in solid film form adjacent the circuit board panel; and   operating a rotary drilling tool for drilling in a dry state through the circuit board panel, including drilling through the lubricating film and at least through the dielectric layer of the panel to form a thru-hole in the panel, in which the lubricating film is formed in a film thickness sufficient to transfer lubricating properties to the rotary drilling tool during penetration of the lubricating film by the rotary drilling tool so that the thru-hole is drilled by the lubricated drilling tool.   
     
     
       2. A process according to claim 1 including removing the lubricating film following drilling of the thru-hole in preparation for forming a conductive metal layer on the wall of the drilled thru-hole. 
     
     
       3. A process according to claim 2 in which the thru-hole is drilled through a panel comprising at least one electrically conductive layer adjacent the dielectric layer. 
     
     
       4. A process according to claim 1 including drilling through the film prior to drilling through the panel. 
     
     
       5. A process according to claim 1 in which the thru-hole is drilled through a panel comprising electrically conductive metal outer layers and dielectric layers adjacent one or more electrically conductive metal inner layers; and in which the thru-hole is drilled through the depth of the panel in preparation for therafter forming a conductive metal layer on the surface of the drilled thru-hole for forming an electrical connection between the conductive outer layers and at least one of the inner layers; and including the step of placing said water soluble dry lubricating film adjacent the conductive metal outer layer prior to drilling the thru-hole through the lubricating film and then through the panel. 
     
     
       6. A process according to claim 5 including the step of forming said conductive metal layer on the surface of the drilled thru-hole. 
     
     
       7. A process according to claim 1 in which the use of said dry lubricating film in the drilling process extends drill life when compared with drilling the same panel without said dry lubricating film. 
     
     
       8. A process according to claim 7 in which a clean thru-hole is produced in the process of using said dry lubricating film, such that further acid treatment of the thru-hole to remove dielectric smear or to etch fibers protruding into the thru-hole is avoided in preparation for forming a conductive metal layer on the wall of the drilled thru-hole. 
     
     
       9. A process according to claim 1 in which the sole means of treating the walls of the drilled thru-hole comprises rinsing the panel with an aqueous rinsing solution having a pH greater than about 7 following the drilling step and prior to forming a conductive metal layer on the surface of the drilled thru-hole. 
     
     
       10. A process according to claim 1 in which the dry lubricating film is impregnated in a water dissolvable porous carrier sheet and is in film thickness on at least one face of the sheet. 
     
     
       11. A process according to claim 1 in which the dry lubricating film comprises a water soluble wax of moderate melting point comprising the principal component of the film, glycol, and a hardener. 
     
     
       12. A process according to claim 11 in which the water soluble wax is a non-ionic surfactant. 
     
     
       13. A process according to claim 11 in which the glycol is dipropylene glycol. 
     
     
       14. A process according to claim 1 in which the dry lubricating film is dried to said solid state with a water soluble hardener contained in the film. 
     
     
       15. A process according to claim 1 in which the lubricating film has a melting temperature lower than the drill tool temperature during the thru-hole drilling step to liquify the lubricating film and coat the drill tool. 
     
     
       16. A process according to claim 1 in which the process is carried out on a printed circuit board panel having an epoxy-glass dielectric material. 
     
     
       17. A process according to claim 1 in which the lubricating film is carried on a sheet of entry material. 
     
     
       18. A process according to claim 1 in which the lubricating film is coated onto the panel. 
     
     
       19. A process according to claim 1 in which the lubricating film is coated in a film thickness on at least one face of a carrier sheet. 
     
     
       20. In a process for drilling printed circuit boards in a dry state which includes drilling a thru-hole in a printed circuit board panel having an electrically conductive layer adjacent a dielectric layer, the improvement comprising placing a water soluble and substantially dry lubricating film in solid film form adjacent one of said layers, and operating a rotary drill to drill in a dry state through the circuit board panel, including drilling through the lubricating film and at least through the dielectric layer of the panel to form a thru-hole in the panel, the lubricating film being formed in a film thickness sufficient to lubricate the drill tool surface with lubricating material transferred from the lubricating film to the drill tool surface while forming a clean thru-hole in the panel, the lubricating film having as its components a water soluble hardener component and a water soluble lubricant combined to form said water soluble substantially dry solid lubricating film. 
     
     
       21. The improvement according to claim 20 in which the water soluble hardener component includes a surfactant. 
     
     
       22. The improvement according to claim 21 in which the water soluble lubricant comprises glycol. 
     
     
       23. The improvement according to claim 20 in which the thru-hole is drilled through a multi-layer panel comprising conductive metal outer layers and a dielectric layers adjacent one or more conductive metal inner layers; and in which the thru-hole is drilled through the depth of the panel in preparation for forming a conductive metal layer on the surface of the drilled thru-hole for forming an electrical connection between the conductive outer layers and at least one of the inner layers; and including the step of placing said water soluble dry lubricating film adjacent the conductive metal outer layer prior to drilling the thru-hole through the lubricating film and then through the panel. 
     
     
       24. The improvement according to claim 20 in which the use of said dry lubricating film in the drilling process extends drill life when compared with drilling the same panel without said dry lubricating film. 
     
     
       25. The improvement according to claim 20 in which a clean thru-hole is produced in the process of using said dry lubricating film, such that further acid treatment of the thru-hole to remove dielectric smear or to etch fibers protruding into the thru-hole is avoided in preparation for forming a conductive metal layer on the wall of the drilled thru-hole. 
     
     
       26. The improvement according to claim 25 in which the sole means of treating the walls of the drilled thru-hole comprises rinsing the panel with an aqueous rinsing solution having a pH greater than about 7 following the drilling step and prior to forming a conductive metal layer on the surface of the drilled thru-hole. 
     
     
       27. The improvement according to claim 20 in which the dry lubricating film is impregnated in a water dissolvable porous carrier sheet and is in film thickness on at least one face of the sheet. 
     
     
       28. The improvement according to claim 20 in which the lubricating film is coated in a film thickness on at least one face of a carrier sheet. 
     
     
       29. The improvement according to claim 20 in which the process is carried out on a printed circuit board panel having an epoxy-glass dielectric material. 
     
     
       30. The improvement according to claim 20 in which the lubricating film has a melting temperature lower than the drill tool temperature during the thru-hole drilling step to liquify the lubricating film and coat the drill tool. 
     
     
       31. A process for drilling a thru-hole in a printed circuit board panel having a dielectric layer made from a material which can be smeared onto other portions of the circuit board panel by the drilling tool and thereby interfere with electrical performance of the circuit board, the method comprising the steps of: placing a water-soluble and substantially dry lubricating film adjacent the panel, in which the dry lubricating film is impregnated in a water-dissolvable porous carrier sheet and is in film thickness on at least one face of the sheet; and   operating a rotary drilling tool for drilling through the lubricating film and at least through the dielectric layer of the panel to form a thru-hole in the panel, in which penetration of the lubricating sheet by the rotary drilling tool causes the lubricant thereon to lubricate the drilling tool so that the thru-hole is drilled by the lubricated drilling tool.   
     
     
       32. A process according to claim 31 in which the use of said dry lubricating film in the drilling process extends drill life when compared with drilling the same panel without said dry lubricating film. 
     
     
       33. A process according to claim 31 in which the lubricating film has as its components a water-soluble hardener component and a water-soluble lubricant combined to form said water-soluble lubricating film. 
     
     
       34. A process according to claim 33 in which the watersoluble hardener component includes a surfactant.